In the reinforced concrete pipe art, it has long been the custom to cast sections by machine, the grid of reinforcing metal rods being placed in a form and the form placed in the machine. The form is so shaped as to produce a bell at the lower end and a spigot at the upper end so that a bell and spigot joint may be created between two adjacent members.
It has also been the custom to form a shallow annular groove in the spigot, to seat the conventional sealing ring, or O ring, just inside the tip flange of the spigot. Upon inserting the spigot in the bell of another member, the sealing ring rolls out of the groove and along the tapered mting surfaces of the bell and spigot until a tight seal is achieved.
It will be understood that there are usually no reinforcing rods in the spigot, that the spigot is of less wall thickness than the main body, and that the spigot is upstanding above the main body when pre-cast and moved from the casting machine into the kiln. The bell on the other hand, while also of reduced wall thickness, is transported into the kiln with its reinforcing mold ring in place so that it has no tendency to lose its integrity.
It has been found that the spigot of such pre-cast reinforced concrete hollow cylindrical sections may tend to slump slightly, or to have flats and that the pre-cast groove extendng therearound is therefore not a true circle and is not of uniform radial distance from the central longitudinal axis of the riser.
It will be understood that it is diffcult enough to obtain the correct rolling movement of a sealing ring out of its groove and into firm contact with the opposed surfaces of bell and spigot, lubrication often proving necessary, without the added hazard of a groove which has been pre-cast with a flat, or bump or other defect.